As is known, early internal combustion engines were controlled to operate while holding fuel-air composition generally constant. Thereafter, oscillatory exhaust gas oxygen (EGO) sensor based fuel control systems were developed, and fuel-air composition could be varied but with resultant torque fluctuations and decreased fuel efficiency.
As part of the California Air Resources Board (CARB) On-Board Diagnostics (OBD-II) regulations, the capability for on-board monitoring of various vehicle sensors, such as the exhaust gas oxygen (EGO) sensor, must be provided by vehicle manufactures beginning with the 1994 model year. Typically, testing is completed during certain steady-state operating conditions which occur during normal vehicle operation. During testing, the vehicle electronic control unit enters a fuel control mode, the duration of which is about 10 seconds long. During this fuel control mode, the control unit alternates the air-fuel ratio between a "rich" ratio and a "lean" ratio at a frequency greater than 1.5 Hz. If the EGO sensor output does not properly respond to the varying A/F ratio, an indicator such as a warning light is energized, notifying the vehicle operator of the problem.
With engine load, engine speed and ignition (spark) timing held relatively constant, engine torque varies with the air-fuel ratio. Air-fuel ratios slightly richer than stoichiometry produce more torque than air-fuel ratios slightly leaner than stoichiometry. As a result, the air-fuel modulations, such as those necessary for the OBD-II test strategy or for oscillatory fuel control, can cause engine torque fluctuations during the test sequence. These torque fluctuations result in engine surges felt by the vehicle operator, thereby affecting driveability.
On multi-bank engine configurations, e.g. V6 and V8 engines with each bank having its own EGO sensor, this problem can be solved by utilizing 180.degree. phasing of the air-fuel modulation between the banks, especially where each bank has an associated catalyst. For example, one bank of the "V" is forced rich, while the other bank is forced lean. The torque increase associated with the rich ratio is in effect canceled by the torque decrease associated with lean ratio. This 180.degree. phasing, however, is not possible on single bank engine configurations, e.g. inline 4- and 6-cylinder engines having a single EGO sensor.
Present practice in electronic engine controllers sets ignition timing based on several factors such as engine speed, intake manifold pressure, average fuel-air composition, and operating temperature, and varies fuel-air composition around stoichiometry. Although there is an ignition timing setting that optimizes engine torque output and hence fuel efficiency, present practice does not control ignition timing as a function of instantaneous fuel-air composition. For purposes of this discussion, fuel-air composition is considered to be a dimensionless measure of the proportional composition of fuel and air in an assumed homogeneous volume. It is often summarized by one of the following quantities: air-fuel ratio, fuel-air ratio, relative air-fuel ratio (.lambda.), equivalence ratio (F.sub.R), and redox potential.
Accordingly, it is desirable to develop an engine control strategy for controlling torque fluctuations during forced fuel excursions, especially on single sensor engine configurations. It is also desirable to develop an engine control strategy for controlling ignition timing based on instantaneous fuel-air composition to achieve optimum engine performance.